DALLAS — The first case of Ebola diagnosed in the U.S. has been confirmed in a man who recently traveled from Liberia to Dallas, sending chills through the area's West African community whose leaders urged caution to prevent spreading the virus.

The unidentified man was critically ill and has been in isolation at Texas Health Presbyterian Hospital since Sunday, federal health officials said Tuesday. They would not reveal his nationality or age.

Authorities have begun tracking down family, friends and anyone else who may have come in close contact with him and could be at risk. Officials said there are no other suspected cases in Texas.

At the Centers for Disease Control and Prevention, Director Tom Frieden said the man left Liberia on Sept. 19, arrived the next day to visit relatives and started feeling ill four or five days later. Frieden said it was not clear how the man became infected.

"I have no doubt that we'll stop this in its tracks in the U.S. But I also have no doubt that — as long as the outbreak continues in Africa — we need to be on our guard," Frieden said, adding that it was possible someone who has had contact with the man could develop Ebola in the coming weeks.

"But there is no doubt in my mind that we will stop it here," he said.

Stanley Gaye, president of the Liberian Community Association of Dallas-Fort Worth, said the 10,000-strong Liberian population in North Texas is skeptical of the CDC's assurances because Ebola has ravaged their country.

"We've been telling people to try to stay away from social gatherings," Gaye said at a community meeting Tuesday evening. Large get-togethers are a prominent part of Liberian culture.

"We need to know who it is so that they (family members) can all go get tested," Gaye told The Associated Press. "If they are aware, they should let us know."

Ebola symptoms can include fever, muscle pain, vomiting and bleeding, and can appear as long as 21 days after exposure to the virus. The disease is not contagious until symptoms begin, and it takes close contact with bodily fluids to spread.

The association's vice president encouraged all who may have come in contact with the virus to visit a doctor and she warned against alarm in the community.

"We don't want to get a panic going," said vice president Roseline Sayon. "We embrace those people who are coming forward. Don't let the stigma keep you from getting tested."

Frieden said he didn't believe anyone on the same flights as the patient was at risk.

"Ebola doesn't spread before someone gets sick and he didn't get sick until four days after he got off the airplane," Frieden said.

Four American aid workers who became infected in West Africa have been flown back to the U.S. for treatment after they became sick. They were treated in special isolation facilities at hospitals in Atlanta and Nebraska. Three have recovered.

A U.S. doctor exposed to the virus in Sierra Leone is under observation in a similar facility at the National Institutes of Health.

The U.S. has only four such isolation units, but Frieden said there was no need to move the latest patient because virtually any hospital can provide the proper care and infection control.

The man, who arrived in the U.S. on Sept. 20, began to develop symptoms last Wednesday and sought care two days later. But he was released. At the time, hospital officials didn't know he had been in West Africa. He returned later as his condition worsened.

Blood tests by Texas health officials and the CDC separately confirmed his Ebola diagnosis on Tuesday.

State health officials described the patient as seriously ill. Goodman said he was able to communicate and was hungry.

The hospital is discussing if experimental treatments would be appropriate, Frieden said.

Since the summer months, U.S. health officials have been preparing for the possibility that an individual traveler could unknowingly arrive with the infection. Health authorities have advised hospitals on how to prevent the virus from spreading within their facilities.

People boarding planes in the outbreak zone are checked for fever, but that does not guarantee that an infected person won't get through.

Liberia is one of the three hardest-hit countries in the epidemic, along with Sierra Leone and Guinea.

Ebola is believed to have sickened more than 6,500 people in West Africa, and more than 3,000 deaths have been linked to the disease, according to the World Health Organization. But even those tolls are probably underestimates, partially because there are not enough labs to test people for Ebola.

Two mobile Ebola labs staffed by American naval researchers arrived this weekend and will be operational this week, according to the U.S. Embassy in Monrovia. The labs will reduce the amount of time it takes to learn if a patient has Ebola from several days to a few hours.

The U.S. military also delivered equipment to build a 25-bed clinic that will be staffed by American health workers and will treat doctors and nurses who have become infected. The U.S. is planning to build 17 other clinics in Liberia and will help train more health workers to staff them.

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